1. Technical Field
The present invention relates to an optical fiber end processing method and an optical fiber end processing apparatus for processing an end of an optical fiber having a plurality of holes around a core part.
2. Description of the Related Art
In recent years, a new optical fiber is focused, which is called a holey fiber or a photonic crystal fiber having a plurality of holes around a core part, and a broad application of the new optical fiber to an optical code for communication and an optical device, etc., is examined (for example, see patent document 1).
FIG. 4 shows a structure of a typical holey fiber. As shown in FIG. 4, a holey fiber 1 is composed of a core part 2; a clad part 3 formed on an outer periphery of the core part 2; and a plurality of holes 4 formed along an axial direction of the core part 2 around the core part 2 of the clad part 3.
When the holes 4 formed on the clad part 3 are opened on an end face of the holey fiber 1, moisture enters into the holes 4 or condensation occurs inside the holes due to temperature variation, thus reducing a mechanical strength of the holey fiber 1 or generating a fluctuation of optical characteristics in some cases.
Further, when optical fibers are connected to each other, using a connecting member such as a mechanical splice or a MT connector, etc., a method of reducing a reflection on a connection end face and a connection loss by filling an interspace between a connection end face of one of the optical fibers and a connection end face of the other optical fiber, with a liquid refractive index matching material. When this connection method is applied to the holey fiber 1 as shown in FIG. 4, and when the holes 4 are opened on the end face of the holey fiber 1, the liquid refractive index matching material enters into the holes 4 from the connection end face. Thus, there is a problem that the refractive index matching material on the connection end face is run out, resulting in causing a large reflection or connection loss.
Further, in a single-core optical connector which requires no refractive index matching material, there is a problem that when the connection end face of the holey fiber 1 is polished, an abrasive or a polishing debris that enters into the holes 4 goes outside after polishing and is caught by the connection end face, thus damaging the fiber end face or deteriorating the optical characteristics due to an interspace generated in this part.
In order to cope with this problem, conventionally the following technique is proposed.
Patent document 1 describes a method for closing pores (holes) of a photonic crystal fiber end, by a closing material with lower refractive index than that of the core part.
Patent document 2 describes a method for collapsing hollow parts (holes) of the optical fiber by heating the optical fiber end and softening the clad part, or a method for sealing the hollow parts by filling the hollow parts with resin, as a method for sealing the hollow parts (holes) of the optical fiber.
Patent document 3 describes a method for melting and closing the holes by arc discharge applied to a position away from the end face of the optical fiber.    Patent Document 1:    Japanese Patent Laid Open Publication No. 2004-4320    Patent Document 2:    Japanese Patent Laid Open Publication No. 2002-323625    Patent Document 3:    Japanese Patent Laid Open Publication No. 2005-24849    Non-Patent Document 1:    “Development trend of Photonic crystal fiber and holey fiber” by Hasegawa, Monthly magazine: “Optronics”, No. 7, pp. 203-208 (2001), issued by Optronics Inc.
The method for sealing the holes by melting the optical fiber itself having the holes by heating described in the patent document 2, has an advantage that there is no problem of aging deterioration, because moisture, etc., is completely prevented from entering into the holes, compared with the method for sealing the holes using the closing material made of resin described in patent document 1. However, the method for melting the end of the optical fiber has a problem that a shape of a core is disturbed and optical loss is easily generated; when a cantilever optical fiber end is heated, a heated portion is bent or expanded due to unbalance of surface tension in a peripheral direction of the optical fiber in a molten state, thus making it difficult to obtain a desirable shape; and the optical fiber end is expanded and is hardly enter into a hole of a ferrule for connector.
Further, according to patent document 3, it is important that two parts of the optical fiber end are respectively fixed to V-grooves, thus enhancing positional accuracy between two V-grooves and suppressing axial shift or angular deviation of the optical fiber between V-grooves as much as possible, so that the bending of the closing part of the holes due to heating and melting is suppressed to be small. However, even if the positional accuracy is enhanced between two V-grooves, there is a problem as follows. Although the optical fiber is placed on the V-groove after coating is removed, there is a possibility that a minute coating refuse remains on a surface of the optical fiber. Then, such a coating refuse adheres to the V-groove portion or a V-groove presser, and the positional accuracy of the optical fiber by a V-groove component is deteriorated, thereby generating deformation and bending of the hole closing part of the optical fiber. Further, not only the coating refuse but also dust floating in the air, and fibers of a cotton-swab for cleaning can also have an adverse influence. In addition, slight deformation and bending are generated in the hole closing part of the optical fiber when being heated and melted, due to incompleteness of the accuracy of the V-groove itself and the V-groove presser, thus inevitably generating a variation in the loss of the optical fiber.
Note that as the method for sealing the holes of the holey fiber, it can be considered that a general optical fiber is fusion-spliced to the connection end face of the holey fiber. However, in this method, the axial shift or angular deviation, and expansion are easily generated in a fusion-spliced part. When such axial shift, angular deviation and expansion are generated, it is difficult to fit the holey fiber into the ferrule, and a surface of the holey fiber is easily damaged during the fitting work, thus posing a problem in workability and reliability. Further, loss in a fusion-spliced part between the holy fiber and the optical fiber is easily excessively generated, if compared with the method for heating and melting the optical fiber between V-grooves described in the patent document 3.